SPECTROSCOPY STUDY OF ARSENITE [As(III)] OXIDATION ON Mn-SUBSTITUTED GOETHITE

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1 Clays and Clay Minerals, Vol. 47, No. 4, , SPECTRSCPY STUDY F ARSENITE [As(III)] XIDATIN N Mn-SUBSTITUTED GETHITE XIAHUA SUN, HARVEY E. DNER, AND MAVRIK AVARIN Division of Ecosystem Sciences, Department of Environmental Science, Policy and Management, University of California at Berkeley, California 94720, USA Abstraet~Mn-substituted was synthesized and the interaction between arsenite (As(III)) and Mn-substituted was investigated by both solution chemistry and X-ray adsorption near edge structure (XANES) spectroscopy. Results indicate that the oxidation of As(III) is favored by Mnsubstituted. This reaction was more sensitive to temperature than to ph. Different reaction mechanisms may account for As(III) oxidation. Since As(III) is more mobile and toxic than As(V), the oxidation reaction of As(liD with Mn-substituted may decrease arsenic toxicity under some conditions. Key Words--Arsenate, Arsenite, Mn-Substituted Goethite, xidation, XANES. INTRDUCTIN The toxicity and bioavailability of arsenic (As) is closely related to its oxidation states. There are two main inorganic species of As in terrestrial ecosystems. The reduced state, arsenite [As(IID], is much more toxic, soluble, and mobile than the oxidized form, arsenate [As(V)] (Duel and Swoboda, 1972; Ferguson and Gavis, 1972). The surfaces of Fe and Mn oxyhydroxides are the major sources and sinks of As and other trace elements (Sung and Morgan, 1981). Recent studies (Waychunas et at., 1993, 1995; Manceau, 1995; Sun and Doner, 1996; Fendorf et al., 1997) indicated that iron oxides can strongly adsorb As(V) by forming binuclear complexes. Mn oxides, on the other hand, are effective oxidants for the transformation of As(III) to As(V) (scarson et al., 1981a; Moore et al., 1990; Scott and Morgan, 1995). Thus, the combined influences of As(III) depletion by oxidation and adsorption in systems composed of Mn and Fe oxides present in terrestrial environments may have a major influence on the toxicity and bioavailability of As (Sun and Doner, 1998). Mn and Fe are closely related in chemical properties and occur together in soils and sediments. Ferric iron in the octahedral positions of oxyhydroxides may be partially replaced by other trivalent metal cations such as AP +, Cr 3+, and Mn 3+ (Schwertmann and Cornell, 1991). Mn-substituted was first synthesized by Stiers and Schwertmann (1985). However, Mn-substituted is not known to occur naturally and studies of Mn-bearing indicate Mn is mainly in the tetravalent oxidation state (Manceau et al, 1992). Manganese can replace <50 mol % of the Fe in the structure of synthetic, c~-feh (Ebinger and Schulze, 1990). Compared to other oxidation states of manganese, aqueous Mn(III) is not stable and is an oxidizing agent powerful enough to evolve oxygen from water. However. Mn(III) can form stable mineral structures, e.g., manganite ('y-mnh), hausmanite (Mn304, containing both Mn(II) and Mn(III), and birnessite (phyllomanganate) (McKenzie, 1989). Manceau et al. (1997) compared the stability of -/-MnH with 13- Mn2 and showed that at ph values <7 the Mn(III) form is a much stronger oxidant than the Mn(IV) form. Iron oxyhydroxides, such as, can adsorb and increase the oxidation rate of adsorbed Mn(II) to Mn(III) by oxygen and stabilize Mn(III) on the surface (Davies and Morgan, 1989). As indicated above, free forms of aqueous Mn(III) are unstable. Little information is available on the redox activities of the structural Mn(III) in where the combined effects of adsorption and changes in oxidation state may be important factors controlling surface reactions. X-ray adsorption near edge structure (XANES) spectroscopy provides a direct method of determining oxidation states of elements such as As and Mn. Different oxidation states of an element have different electron bonding energies. In an X-ray adsorption spectrum, the energies of the edge-peak increase with increasing oxidation state. The edge shift between As(III) and As(V) in XANES spectra is great enough for determining As(Ill) and As(V) semiquatitatively in a soil sample with unknown As oxidation states (Foster et al., 1995). Also, the oxidation states of Mn in soils can be observed directly by XANES (Schulze et al., 1995), although the accuracy of the method may be limited. In this paper, Mn-substituted was synthesized and the interaction between As(III) and Mn-substituted was studied by XANES spectroscopy and solution chemical methods. In addition, the effects of environmental conditions such as ph and temperature were also investigated. Copyright , The Clay Minerals Society 474

2 Vol. 47, No. 4, 1999 Arsenite oxidation on Mn-substituted 475 Table 1. Some properties of prepared and Mn-substituted samples. Pure 1% Mn 5% Mn Property Surface area I (m2/g) phpz~c Total Mn(%, Mn/(Mn + Fe)) M NaC1 soluble Mn (% of total Mn) M CuC12 extractable Mn 3 (% of total Mn) Color 2.5y 7/8 2.5y 5/8 5y 4/3 5y 3/2 EGME method (Heilman et al., 1965). 2 Acid-base titration (Sposito, 1981). 3 Adapted from Traina and Doner (1985). METHDS AND MATERIALS Preparation and characteristics of Mn-substituted Mn-substituted was synthesized by the method of Schwertmann and Cornell (1991) using analytical grade regents, except those mentioned specifically. A quantity of 175 ml of 2.0 M NaH was added to 50 ml of a mixed solution of Fe(N3)3.9H20 and Mn(N3)z.4H20 having a total Fe + Mn concentration of 0.53 M and Mn/(Mn + Fe) ratios to 0.00, 0.01, 0.05, and The products were centrifuged, washed, and stored in polyethylene bottles in 250 ml of 0.3 M NaH solution at 60~ for 15 d. The samples were washed and centrifuged again several times with deionized water, placed in molecular porous membrane tubing (VWR Scientific, Catalog No ), and dialyzed in deionized water until the electric conductivity of the dialysis water equaled that of the deionized water. The samples were oven dried at 60~ then ground with a mortar and pestle and passed through a 37 Ixm sieve. All solid products showed X- ray diffraction patterns identical to pure. The total Mn in Mn-substituted was determined by dissolving the solid products in 6.0 M HC1 at 70~ and analyzing by inductively coupled plasma (ICP) spectroscopy. The NaC1 and CuC12 extractable Mn from Mn-substituted were determined by addition of 10 ml 0.01 M NaC1 or 0.05 M CuC12 (modified from Traina and Doner, 1985) solution to 50 ml centrifuge tubes which contained 0.10 g solid samples. The tubes were shaken for 1 h, centrifuged, and filtered. The Mn in the supematant was determined by ICP spectroscopy. Some properties of the products are listed in Table 1. XANES study The Mn oxidation state in Mn-substituted was examined by using the sample with lowest Mn concentration (1% Mn) to minimize self-absorption effects (Schulze et al, 1995). Mn(II) and Mn(IV) standards were prepared by mixing MnS4 (analytical agent, ground) and Mn2 (99.5% Mn2, Matheson Coleman & Bell, ground) with boron nitride powder to give solid mixtures which contained the same amount of Mn as 1% Mn-substituted. The powder samples were mounted in a mm milled slot cut in a Teflon sheet, and sealed with thin Kapton polymide film to minimize X-ray absorption. Mixtures of As(III) and As(V) treated pure were prepared for XANES spectral analysis. This was done by adding 300 ixl solution of various concentrations of arsenate (Na2HAs4) and arsenite (NaAs2) at ph 6.5 to 50 mg of pure to give a total adsorption density of 150 Izmol As g 1. After 2 h the paste samples were mounted in the milled Teflon slots as described above. The sample cells were sealed with thin Kapton polymide film to prevent moisture loss and to minimize X-ray absorption during scans. Preliminary experiments showed that the adsorption maximums of different Mn-substituted for As(V) and As(III) were all above 180 p, mol As g-~ at ph 6.5. This was estimated by fitting the Langmuir equation to the data and extrapolation to an adsorption maximum. Thus, all treatments were undersaturated with respect to the adsorption maximum. Mn-substituted suspensions for kinetic study were prepared as follows: 0.05 g of Mn-substituted was added to 5.0 ml of 0.01 M NaC1 solution, followed by the addition of 0.1 ml of 75 ~mol/ml As(III) producing 150 Ixmol As(III) g 1 solid. The ph was adjusted to 6.5 with 0.1 M HC1. Samples were then mixed continuously to 10 d in capped centrifuge tubes on a shaker. Following the different aging periods, the mixtures were centrifuged and sediment-pastes were collected. The paste samples for XANES spectral analysis were prepared at different time intervals ranging from 2 h to 5 d before scanning. To examine the effects of dry conditions on As(III) stability, samples were prepared as follows: 0.1 ml of 75 Ixmol ml 1 As(III) was added to 1.0 ml of a 5% (w/w, 5 g solid in 100 g solution) Mn-substituted suspension in a 0.01 M NaC1 solution producing 150 txmol As(III) g ~ solid. The ph was adjusted to

3 476 Sun, Doner, and avarin Clays and Clay Minerals of As(0) was taken to be kev in transmission. The Mn XANES spectra were collected from 6,000 to kev by using Mn metal foil to calibrate energy (half-height edge energy of Mn metal was taken as kev). The Ge-fluorescence detector was used to collect spectral data. All spectral data are presented without smoothing , Energy (kev) Figure 1. Normalized K XANES spectra of Mn in l% Mnsubstituted, Mn z, and MnS with 0.1 M HCI, mixed and aged in air-dry condition to 10 d. The air-dried samples for XANES spectral analysis were prepared by using the same procedure as above. ph and temperature effects on As(IID oxidation by Mn-substituted were studied by adding 0.05 g of Mn-substituted to 5.0 ml of 0.01 M NaCI solution. To this suspension, 0.1 ml of 75 ~mol ml -~ As(III) was added to produce 150 tzmol As(III) g ~ solid. The ph was adjusted to 5.0, 6.5, and 8,0 with 0.1 M He1 or NaH. Samples were then mixed continuously to 10 d in capped centrifuge tubes on a shaker. The treatments at ph 6,5 were aged at 25, 45, and 65~ to 10 h in capped centrifuge tubes. Following the different aging periods, the mixtures were centrifuged. Sediment-paste samples were collected for XA- NES spectral study. Replicates of the above treatments were used to determine extractable Mn(ll) produced from the reaction with As(II1). The samples were centrifuged and the superuatant was taken for Mn analysis. Then ml of 0.05 M CuC12 was added to the mineral residue, shaken for 1 h and centrifuged again. The supernatant was taken, combined with the previous collection and all Mn released to the solution phase was determined by ICR The XANES spectra of both As and Mn were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) beam line 4-3 equipped with a wiggler. The monochromator used Si(220) crystals and was detuned -50%. Arsenic XANES spectra were collected from 11,650 to kev by using a Lyric-type fluorescence detector with a 6 mm Ge-filter. Energy was calibrated by the XANES spectrum of As(0) with the ion chamber positioned along the beam path and behind the primary sample. The half-height edge energy RESULTS AND DISCUSSIN Manganese substituted has a structure nearly identical to pure according to X-ray diffraction data (data not shown). A previous study (Cornell and Giovanoli, 1987) indicated that, in alkali condition (ph = 11-13) and with Mn additions of as much as 15 mole %, Mn-substituted was the sole reaction product. With an increase in Mn substitutions (Table 1), the color of samples became darker and the point of zero net proton charge (PNPC) decreased, Unsubsfituted was yellow, but as little as 1 mole % Mn produced an olive color, and highly substituted was gray or black. Figure 1 shows Mn K XANES spectra of Mn2, MnS4, and Mn-substituted and these spectra are consistent with Schulze et al. (1995). Schulze et al. did not examine Mn-substituted. Mn(II) (MnS4) K XANES spectrum was characterized by an edge-peak at kev. For Mn(IV) (Mn2), the edge-peak was at kev whereas for Mn-substituted the edge peak was kev. The position of the Mn edge-peak for Mn-substituted was between that of Mn(II) and Mn(IV), suggesting the main oxidation state of Mn in Mn-substituted was Mn(IlI). This is compatible with the suggestion of Schwertmann and Cornell (1991). They proposed that although Mn(II) is used in preparing of Mn-substituted, it is oxidized to Mn(III) by atmosphere oxygen under alkaline conditions and incorporated into the structure as Mn(IlI). Arsenic K XANES spectra of pure treated with 150 Ixmol g ~ arsenic at different As(V)/As(III) ratios are shown in Figure 2. This figure shows that the 100% As(III)-treated has an edge-peak at kev whereas the 100% As(V)-treated has an edge absorption peak at kev. As the mole fraction of As(III) increased, the intensity of the kev peak increased whereas the kev peak decreased gradually. These differences were used in a comparative way to identify the oxidation transformation from As(III) to As(V) on surfaces. Figure 3 shows the reaction of As(IID on Mn-substituted at ph 5.0 and 8.0. At room temperature, no apparent oxidation occurred on pure after 10 h. The oxidation of As(III) to As(V) by Fe(III) is a thermodynamically favorable reaction at low ph conditions (scarson et al., 1980). However, they found that the conversion of As(III) to As(V) by synthetic Fe(III) oxyhydroxide did not occur within 72 h

4 Vol. 47, No. 4, 1999 Arsenite oxidation on Mn-substituted / ~. ~ flas(llll) flas(vi) / /,~.g ooo c , , i i Energy (key) Figure 2. Normalized As K XANES spectra of As(III) and As(V) mixtures at different As(III)/As(V) ratios on. i a.ph=5.0 ~t/ 5% Mn,...~_.~ // / / 1% Mn j / 0%.Mn, ~ i t.57 1t 873 b. ph=8.0 11,873 I! I at neutral ph values, suggesting the reaction is kinetically controlled. xidation of As(III) increased with increased Mn substitution in. This indicates that Mn-substituted is a stronger oxidant for the oxidation of As(III) than pure. There were no apparent significant differences between the amount of As(III) oxidized at ph 5.0 and 8.0. The absence of a strong ph influence is predicated from the acid-base chemistry of H3As 3 (pka = 9.3) (Scott and Morgan, 1995). In the ph range studied here, the concentration of the H3As3 ~ species remains nearly constant. At room temperature, the oxidation of As(III) on Mn-substituted was slow. Figure 4 shows the reaction of As(III) on 5% Mn-substituted as a function of time. No apparent oxidation was observed within 3 d in either suspension or air-dry conditions. By day 5, comparing results in Figure 2 with those in Figure 4, we estimate 20% or less of As(III) was oxidized to As(V). An extended reaction period along with a more detailed spectral analysis may reveal differences in oxidation rates between suspension and air-dry samples. Figure 5 shows that the oxidation of As(III) on Mnsubstituted was greatly enhanced at elevated temperatures. At 65~ nearly all As(III) was oxidized to As(V) on -substituted after 10 h. n 5% Mn-substituted, the As(IlI) peak was dominant at 45~ whereas the As(V) peak dominated at 65~ Even on pure, As(III) oxidation occurred at 65~ after 10 h. Increasing temperature may favor an increase in As(liD oxidation in several ways. For example, the desorption and transfer rate of As(II1) to sites near Mn may increase, thus increasing the "o ~N 0= 50/. M. 1% Mn! I 0% Mn I I t Energy (key) Figure 3. Normalized As K XANES spectra of As(III) adsorbed on Mn-substituted 10 h after As treatment, a) ph = 5.0, b) ph = 8.0. electron transfer rate. However, it may not be necessary for As(III) to come in close contact with Mn, but the oxidation potential of the entire solid phase may be more favorable for As(V) formation. Also, with increasing temperature, the activation energy of the As(III) oxidation reaction may decrease thereby making the reaction faster, such as the case with pure. Figure 6 shows the total extracted Mn(II) by CuC12 from Mn-substituted after reacting with As(III) at 65~ Note in Figure 5, nearly all of the As(III) (150 ixmol g i solid) was apparently oxidized to As(V) on -substituted after 10 h. If we assume the reaction: As(III) + 2Mn(III) = As(V) + 2Mn(II), (1) then 300 txmol Mn(II) g 1 solid will be produced.

5 478 Sun, Doner, and avarin Clays and Clay Minerals a.suspension 11,873 a. 45~ " G).N t~ E z ~ \ 5% Mn 3 day J 1 day j I I I I % Mn 0% Mn I! b. Air dry b. 65~ tl.876 C _= ".N E td.,j 1 I I I I Figure 4. Normalized As K XANES spectra of As(Ill) adsorbed on 5% Mn-substituted as a function of time. a) Suspension, b) Air-dry. However, the extracted Mn(II) from - was only -30 p~mol g-l, much less than calculated from reaction (1). For 1% Mn- --30% of As(III) was oxidized to As(V) as estimated by comparing Figures 5 and 2, The Mn(H) produced from reaction (1) will be 90 p~mol g l solid, which is --80% of the total Mn in 1% Mn-. If this amount of Mn(II) is not extractable and remains in the solid phase, it will change the Mn XANES spectrum. However, there is no apparent difference in Mn oxidation states in 1% Mn- before and after reaction with As(Ill) (Figure 7). r- e".n g 5% Mn 1% Mn 0% Mn I I I Figure 5. Normalized As K XANES spectra of As(III) reacted with different Mn-substituted as a function of temperature, a) 45~ b) 65~ The results suggest that the oxidation of As(III) to As(V) was favored by Mn-substituted. Previous research indicated that in a natural, open system (scarson et al., 1981b) as well as in controlled environments (Scott and Morgan, 1995), Mn in Mn oxides is able to increase the rate of As(liD oxidation. Davis and Morgan (1989) found that the oxidation of Mn(II) to Mn(III) is very slow at ph values <8.5, but the rate increases through surface interactions with. They found that oxidation is strongly temperature dependent. Molecular oxygen is assumed to be the terminal electron acceptor. Because of our experimental conditions, Mn(lI) formation and re-oxidation to Mn(III) could not be detected.

6 Vol. 47, No. 4, 1999 Arsenite oxidation on Mn-substituted N Calculated from: As(Ill) + 2Mn(lll) = As(V) + 2Mn(II) g.~ 200 o ~ Extracted. 5~176 Mn 1% Mn-substituted / ' ~ - " ' ~ I~ 100 I% Mn After reaction // / [ 0% Mn 0 ~ Figure 6. Mn released from Mn-substituted after reaction with As(III) at 65~ Based on the above results, mechanism for electron transfer cannot be uniquely identified. Since Mn(III) ([Ar]3d 4) has one electron less than Fe(III) ([Ar]3dS), Mn-substituted may act as a p-type semiconductor (Bockris and Reddy, 1973). Thus, electrons accepted from As(Ill) oxidation by Mn-substituted may be de-localized over the solid instead of associated with a specific surface atom. nly those electrons of Mn(II) on the surface may be desorbed and released to the solution. This may account for the relatively small amount of Mn(II) extracted from the Mnsubstituted shown in Figure 6. In terms of charge and size, Mn(II) atoms may not be stable in the structure and thus re-oxidize to Mn(III), with 02 acting as the electron acceptor. This re-oxidation may be a fast reaction because no change of oxidation state of Mn was observed. ther possible mechanisms may also explain our results. For example, it was observed for V that the oxidation rate of vanadyl(iv) by 02 is increased by its adsorption on ~- A1203 and Ti2 (anatase) (Wehrli and Stumm, 1988, 1989). This was attributed to a more favorable electron environment as a result of an inner-sphere complex formation (Wehrli et al., 1989). Their results did consider isomorphous substitution in the mineral. More experimental data are needed to identify the process. Regardless of the mechanism for As(liD oxidation, Mn substitution in and temperature are important in controlling the process. CNCLUSINS Arsenite [As(Ill)] can be oxidized by Mn-substituted. This reaction was more sensitive to temperature than to ph. An increase in Mn substitution in results in an increase oxidation rate of As(Ill) to As(V). Different mechanisms may explain this observation. The surface of Mn-substituted possibly acts like a semi-conductor in which electrons are Before reaction / I i i i , ,570 6,580 Energy (kev) Figure 7. Normalized K XANES spectra of Mn in 1% Mnsubstituted before and after reaction with As(III) at 65~ de-localized in the entire mineral instead of only associated with surface atoms. Also, changes in the local environment of As(Ill) possibly produces the formation of an inner-sphere complex with Mn-substituted enhanced oxidation. xygen may be the electron acceptor in either case. Since Mn-substituted can both convert As(Ill) to As(V) and strongly adsorb As(V), under certain conditions the reactions may be useful in decreasing As toxicity in environments contaminated by As(Ill). ACKNWLEDGMENTS Financial support from Water Resources Center of University of California is appreciated. REFERENCES Bockris J.'M. and Reddy, A.K.N. (1973) Modern Electrochemistry 2. PlenmrdRosetta Publication, New York, Cornell, R.M. and Giovanoli, R. (1987) Effect of manganese on the transformation of ferrihydrite into and jacobsite in alkaline media. Clays and Clay Minerals', 35, Davis, S.H.R. and Morgan, J.J. (1989) Manganese (II) oxidation kinetics on metal oxide surface. Journal of Colloid and Interface Science, 129, Deul, L.E. and Swoboda, A.R. (1972) Arsenic toxicity to cotton and soybeans. Journal of Environmental Quality, 1, Ebinger, M.H. and Schulze, D.G. (1990) The influence of ph on the synthesis of mixed Fe-Mn oxide minerals. Clay Mineralogy, 25, Fendorf, S.M., Eick, J., Grossl, RR., and Sparks, D.L. (1997) Arsenate and chromate retention mechanisms on. 1. Surface Structure. Environmental Science & Technology, 31, Ferguson, J.E and Gavis, J. (1972) A review of the arsenic cycle in nature waters. Water Resources, 6,

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